1. Technical Field
The present invention relates to mechanical linkages for reducing lateral roll and axle beam rotation.
2. Background Art
Suspension systems function to isolate the sprung mass of the vehicle from the unsprung masses comprising wheel and axle assemblies. Shock loading and vibration are reduced by springs and dampers. Mechanical linkages may be provided to improve the stability, steering and general handling performance of the vehicle.
Anti-roll bars, or stabiliser bars, are used to reduce the tendency of the vehicle body to roll or rotate about a longitudinal axis in a turn. During cornering, a typical passenger car may roll from 3-8° per g toward the outside of the turn. Too much roll is undesirable because it places difficulties for the driver to make accurate observations on the road conditions and the motion of the vehicle. Excessive roll may also interfere with steering performance and/or the stability of the vehicle.
An anti-roll bar is connected between left and right side of a suspension in such a manner that it is only articulated during opposite wheel motion i.e. suspension/vehicle roll, i.e. one wheel moving into bump (closer to vehicle body) and another into rebound (away from the vehicle body). The stiffness of the anti-roll bar reduces the amount of roll for a given change in vertical forces, or increases the load transfer for a given roll angle. Its stiffness is tuned to achieve desired levels of roll, and change the distribution of roll stiffness between front and rear axle.
Several ways to construct an anti-roll bar, or an anti-roll stiffness for a suspension are known from prior art. The most common set-up consists of a bar or an essentially wide U-shaped bar or beam mounted in front or behind a suspension. The arms of the U are connected to the opposite sides of the suspension, and the centre section to vehicle frame, sub-frame, or chassis. In a typical arrangement, the arms of the anti-roll bar are connected to a suspension member (for example, control arm, suspension strut, a knuckle, or axle) using a link with either bushings or ball joints at each end. The centre section is usually connected to the chassis using purpose-built bushings, sometimes referred to as D-blocks due to the characteristic shape of their cross-section. The shape is dictated by the fact that it first must be possible to mount the bush on the bar, and secondly because it is normally affixed on a flat surface by means of a clamp.
In the typical installation described above, the arms of the anti-roll bar are essentially loaded in bending, and the centre section essentially in torsion. Due to package constraints the centre section will sometimes be subjected to a combination of torsion and bending loads. On a live (or a dead beam) axle, the installation is often reversed, such that the centre section is connected to the axle, and the arms to the chassis.
Anti-roll bars are generally attached to suspension members using links of varying length, commonly referred to as drop links. The purpose of the drop link is to allow the anti-roll bar tip and suspension member to follow different trajectories during wheel travel, such that roll resistance is provided without introducing side loads on either the anti-roll bar or the suspension member.
Leaf spring live axle, also referred to as Hotchkiss suspension, is rarely used in modern passenger car applications, but it is still a common solution on light trucks and commercial vehicles. Its popularity is due to its cost-effectiveness and low overall system weight, resulting in a higher payload for a given gross vehicle weight than many other systems. Further, leaf spring divides the load to two attachment points on a chassis, which is favourable from durability point of view.
However, leaf spring suspensions suffer from the inherent disadvantage of relatively low wind-up stiffness, i.e. angular articulation of the axle (in side view) under driving/braking forces. The driving or braking torque is reacted on the leaf springs through axle housing. Under severe braking or acceleration, the leaf springs will presume an s-shape deformation. This articulation is undesirable, because it deteriorates road holding and traction through uncontrolled axle motion. Further, it has adverse NVH (noise, vibration and harshness) effects and poses additional stress on the driveshaft, which in extreme cases may lead to axle breakage if the joints are over-articulated. Thus it is desirable to minimise wind-up.
For a given vertical spring rate, it is possible to improve wind-up stiffness by making springs longer. However, this requires package space and adds mass to the vehicle. Also, this option will generally not be available on an existing vehicle platform where modification is desired, such as increased engine torque output. Tramp rods, also referred to as torque arms or radius arms, have historically been applied in vehicles equipped with a leaf-sprung live axle (normally rear) that produce high torque, particularly in high performance and racing car application. Tramp rod or rods—usually a single one or a pair per axle—are most commonly installed such that they extend parallel or nearly parallel to the leaf spring in forward direction. The tramp rod counteracts wind-up of the axle and differential caused by high torque loads. The length and position of the tramp rods have to be selected carefully, such that the installation will not be ‘fighting’ the motion of axle housing under normal wheel travel.
Another conventional means to improve wind-up performance is staggered installation of the dampers. In such arrangement, one of the dampers is located in front of the axle and inclined forward of the axle, while the other is behind the axle inclined rearward. Such arrangement is often used on overslung suspension. It does not increase wind-up stiffness, but generally results in better wind-up performance, because the articulation during transients is reduced. However, the asymmetric installation produces asymmetric motion ratios, leading to asymmetric vehicle response. Especially in laden condition the rearward damper will be less effective. Except for wind-up control, staggered installation is thus undesirable.
A further desired characteristic of a suspension system is compliance understeer. On a rear suspension, compliance understeer is achieved when a cornering (lateral) force applied on outside corner wheel causes the axle to toe-in. In practice, this is difficult to achieve on a conventional leaf spring suspension. Panhard rod or Watt's linkage have previously been applied, but it is often difficult to find either the package or, on a vehicle with ladder frame, a sufficiently stiff mounting point.
These problems, and possibly other problems, are addressed by applicants as summarized below.